The invention relates to a method for controlling an automatic shutdown and start-up process of a drive unit in a motor vehicle.
In order to reduce fuel consumption and pollutant emissions, the current trend is to develop methods and systems (and to some extent they have also been already installed) that automatically shutdown the internal combustion engine of a motor vehicle under certain conditions and/or in the presence of predetermined shutdown conditions and automatically re-start the internal combustion engine in the presence of predetermined start-up conditions and/or start requests. Such methods and systems or, more specifically, such start and stop devices lend themselves well predominantly to city traffic, in order to reduce the consumption of fuel. This is because in city traffic the vehicle often comes to a standstill at traffic lights or owing to the traffic and because it is not necessary to run the internal combustion engine.
Hence, DE 101 61 343 A1 discloses an automatic stop and start-up control device for an internal combustion engine. In this case the control device executes corresponding measures for shutting down the internal combustion engine, when all of the shutdown conditions are met. For example, the speed of the vehicle has to be below a predetermined limit value. In the case of manual transmissions no gear may be engaged, and in the case of vehicles with automatic transmissions the gear shift position of the transmission must be in a neutral position.
In addition, DE 100 30 290 A1 also discloses a method and a system for automatically shutting down and starting up again an internal combustion engine. In this case an automatic shutdown process is inhibited, when the charge state of the battery of the vehicle is below a predetermined lockout threshold. As additional shutdown conditions, it is also checked whether the speed of the vehicle is below a predetermined limit value, whether the transmission is in the idling position, and whether the accelerator pedal is in the rest position.
Finally DE 10 2007 003 289 A1 discloses a method for controlling an automatic shutdown and start-up process of a drive unit in a motor vehicle that is equipped with an air conditioning system. In this case the drive unit, which has been automatically shut down, is automatically started, when a start request is made. The activation of a start request occurs, for example, as a function of an operating parameter of the air conditioning system, in particular as a function of the present evaporator temperature.
The trend in modern motor vehicles provided with an air conditioning system and a start and stop system is to predetermine, for example, as a function of the outside air temperature, an evaporator temperature threshold value, at which or above which a start request for initiating an automatic start process of a drive unit, which has been automatically shut down, occurs.
The evaporator temperature threshold value is supposed to prevent the evaporator that is warming up from dragging moisture and/or odor into the interior of the motor vehicle when the drive unit is shut down. In order to prevent extremely short stop periods of the drive unit, an additional evaporator temperature threshold, which is somewhat lower than the start threshold, was also introduced. This additional evaporator temperature threshold causes a shutdown inhibition, when at the time that the shutdown process of the drive unit is initiated, the current evaporator temperature already exceeds this additional threshold.
The drawback with such a method is that despite the undershooting of the additional threshold (for example, because of the blower fan output or a high internal temperature) only a very short stop period is achieved due to the different boundary conditions. In contrast, even if this additional threshold is exceeded, the stop period could be long enough under other boundary conditions. Moreover, with such an inflexible threshold other climate parameters, such as the state of charge of an accumulator evaporator that may be used, are not considered as the start requesters.
The object of the invention is to provide an improved method that is intended for controlling an automatic shutdown and start-up process of a drive unit in a motor vehicle by which a shutdown process is initiated only if a sufficient standstill period (until a system dependent start requester is reached) can be guaranteed.
This and other objects are achieved by a method for controlling an automatic shutdown and start-up process of a drive unit in a motor vehicle, which is ideally equipped with an air conditioning system, by way of a start and stop device, by which the drive unit is automatically shut down, when the motor vehicle comes to a halt, if predefined shutdown conditions are met, and by which a drive unit, which has been automatically shut down, is automatically started up, if at least one start request is made. In order to be able to ensure a sufficient standstill time until a system dependent start request occurs, the invention provides that before the initiation of an automatic shutdown process of the drive unit, a possible stop duration of the drive unit is predicted, taking into consideration the present operating variables of the vehicle, the state variables, and/or other parameters. Then, depending on the predicted stop duration, an automatic shutdown process is allowed or inhibited when all other shutdown conditions are met.
This novel method has the advantage that when all other shutdown conditions are met, an automatic shutdown process is initiated, when the predicted stop duration is at least not less than a predetermined stop duration limit value that may also be variable, if desired. If, however, the predicted stop duration is less than the optionally freely applicable stop duration limit, then, however, no automatic shutdown process is allowed.
This method can prevent the driver from becoming unsure due to frequent re-start-ups of the drive unit after a short standstill duration. This method also makes it possible to achieve a fixed minimum stop duration under almost all circumstances.
Ideally, the stop duration is predicted by use of a mathematical model, because virtually all physical processes that are relevant for the possible stop duration can be simulated by developing a model.
The advantage of this approach is that the stop duration up to the occurrence of a predetermined and defined start request is predicted. Since driver-sided start requesters are difficult to predict, the stop duration until the occurrence of a system dependent start request (for example, a start request because a predetermined evaporator temperature has been exceeded) is predicted.
If the motor vehicle having the start and stop device is equipped, for example, with an air conditioning system and if the method for controlling an automatic shutdown and start-up process of the drive unit is configured such that when the air conditioning system is switched on, a system dependent start request occurs as a function of overshooting a threshold value of the evaporator temperature of an air conditioning system evaporator, then the stop duration until the occurrence of an evaporator temperature dependent start request can be predicted before the automatic shutdown of the drive unit, wherein the evaporator temperature dependent start request is made when a predetermined evaporator temperature threshold value is reached or exceeded. This means that before the initiation of the shutdown process which is possible in principle, it is determined how long the drive unit could stay shut down, until the evaporator temperature increases so far that it will presumably reach or exceed the predetermined maximum evaporator temperature threshold value. If the determined expected stop duration is long enough, then the automatic shutdown process is allowed. If this duration is very short, then the automatic shutdown process is suppressed.
In order to be able to determine, or more specifically predict, the stop duration until the occurrence of the evaporator temperature dependent start request, a plurality of current operating variables of the vehicle, state variables and/or other parameters can be considered. In particular, the operating variables of the air conditioning system, the operating parameters of the air conditioning system, and/or at least one climate parameter characterizing the climate state of the interior of the vehicle and/or a climate parameter characterizing the climate state of the immediate vehicle environment should be considered. As a result, the physical modeling for predicting the standstill duration until the occurrence of the evaporator temperature dependent start request, which immediately brings about an automatic start of the drive unit, can be based on at least one or more of the following input variables or, more specifically, characteristics: (1) activity of a climate function, (2) current evaporator temperature, (3) evaporator temperature threshold value, at which a start request occurs, (4) current outside temperature, (5) inside temperature, (6) current blower fan output of a front blower fan, (7) current blower fan output of a rear blower fan (if present), (8) the melting point of any refrigerant accumulator medium that is used, (9) the temperature dependent enthalpy of fusion of any refrigerant accumulator medium that is used, (10) the enthalpy of the evaporation of the water accumulated in the air conditioning system evaporator, (11) rate of the enthalpy increase when the air conditioning system evaporator is under load (that is, the phase transformation of any accumulator medium that is used and/or the accumulation of water in the evaporator), (12) thermal capacity of the air conditioning system evaporator, (13) mass of the evaporator, (14) parameters for estimating the air inlet temperatures at the air conditioning system evaporator, (15) conversion curve for converting the expression of the percent amount of air into an air mass flow expression, and/or (16) the specific thermal capacity of the air.
The inventive method and its advantageous embodiments can be carried out by way of an implemented algorithm or a corresponding arrangement of modules in a control unit that is provided for this purpose, in particular in an engine control unit or an air conditioning system control unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.